Update date & time function v2

[mattyrob]

Description

This PR is a backport of a major upgrade to the core date and time functions. Fixes #903

Motivation and context

The original date and time functions originate back in the days of PHP4. Upstream in WordPress there has been a major piece of work updating these functions to make them more reliable.

How has this been tested?

This PR include around 60 changeset backports, with each local testing has been done to ensure unit tests are still passing. This enabled several other changes sets to be identified and back ported as they were needed for particular changeset as part of this work. More details of the changes sets reviewed and back ported is given below.

43380 - conflict
43387 - clean
43434 - conflict
43562 - clean
43591 - clean
43594 - clean
43599 - null
43610 - null
43703 - null
44592 - clean
44710 - clean
44809 - conflict
45375 - conflict
45377 - clean
45379 - conflict
45424 - conflict
45556 - conflict
45850 - conflict
45853 - conflict
45855 - clean
45856 - clean
45876 - conflict
45882 - conflict, better with 43380
45883 - clean
45887 - conflict
45900 - clean
45902 - clean
46083 - conflict
46569 - clean, better with 45901 and 45914
46578 - conflict, better with 46154
46579 - conflict
46580 - clean
46583 - clean
46584 - clean
46756 - not needed 5.2.0 code
46757 - not needed 5.2.0 code
46774 - not needed 5.2.0 code
46785 - not needed - test file not in CP
46795 - conflict
46796 - clean
46862 - conflict
46863 - not needed, branch
46958 - clean
46973 - conflict
46974 - not needed, patches function introduced in 5.2.0
46977 - not needed, patches function in branch introduced in 5.2.0
46981 - not needed, patches testing of function introduced in 5.2.0
46982 - not needed, patches testing in branch of function introduced in 5.2.0
47073 - conflict
47076 - clean
47078 - conflict
47079 - conflict
47080 - conflict
47098 - conflict
47127 - not needed, backport to branch
48911 - conflict, needs 45908, 45911 and 47118
48912 - conflict, needs 47287
48918 - clean
48920 - not needed, for branch
48921 - not needed, for branch
48922 - not needed, for branch
48924 - conflict
48926 - not needed, for branch
51905 - conflict
51906 - conflict
51950 - conflict

Screenshots

N/A

Types of changes

• New feature / enhancement

[KTS915]

I don't have anything substantive to add, but just want to say: "Wow, @mattyrob , great work!"

[mattyrob]

I don't have anything substantive to add, but just want to say: "Wow, @mattyrob , great work!"

Thanks @KTS915 - but this is just a re-opening of a PR we merged yesterday that broke some tests so was reverted. Weird that it is passing everything again on the PR checks.

[joyously]

So, why does it say zero files changed? Is it already merged?

[mattyrob]

@joyously - good spot, thanks for pointing that out. I needed to revert the reversion to restore the changes in the original PR.

[bahiirwa]

We should close this for ClassicPress/ClassicPress#1116

[mattyrob]

We should close this for ClassicPress/ClassicPress#1116

Agreed ClassicPress/ClassicPress#116 looks cleaner to fix up.